Context variables in Python

Question

Suppose that I have a function in my Python application that define some kind of context - a user_id for example. This function call other functions that do not

Answer 1

Essentially what you're looking for is a way to share a state between a set of function. The canonical way to do so in an object oriented language is to use a class:

class Foo(object):
    def __init__(self, operation, user=None):
        self._operation = operation
        self._user_id = user.id if user else None

    def f1(self):
        print("in f1 : {}".format(self._user_id))
        self.f2()

    def f2(self):
        print("in f2 : {}".format(self._user_id))
        self.f3()

    def f3(self):
        print("in f3 : {}".format(self._user_id))


 f = Foo(operation, user)
 f.f1()

With this solution, your class instances (here f) are "the context" in which the functions are executed - each instance having it's own dedicated context.

The functional programing equivalent would be to use closures, I'm not going to give an example here since while Python supports closures, it's still first and mainly an object language so the OO solution is the most obvious.

And finally, the clean procedural solution is to pass this context (which can be expressed as a dict or any similar datatype) all along the call chain, as shown in DFE's answer.

As a general rule : relying on global variables or some "magic" context that could - or not - be set by you-dont-who-nor-where-nor-when makes for code that is hard if not impossible to reason about, and that can break in the most unpredictable ways (googling for "globals evil" will yield an awful lot of litterature on the topic).